Are we "rolling 13s" and getting thousand year storms every year?
|NOAA April 2011 Precipitation Anomaly|
The contour plots below are taken from Theory of the hydraulic jump and backwater curves. These studies of historical storm records were used to inform design decisions for the Miami Valley Conservancy District's retarding basins and channel improvements following the 1913 floods. My previous post has pictures of the hydraulic jump below Huffman Dam in operation.
My question to Dr Curry about what value high-fidelity (read: relatively expensive to run and analyze) climate simulations have for decision makers was motivated by reading up on infrastructure projects like the retarding basins and channel improvements in the Miami Valley. I think it would be interesting to take a look at a historical project like this that included rudimentary analysis of climate (weather event frequency and magnitude) in its design, and say, "here's how it would be informed differently using modern tools."
The design philosophy taken by the engineers working for the Miami Valley Conservancy District was to design for the worst possible case (historical records from Europe were also considered since they went back further and more reliably) plus roughly twenty percent margin due to the inherent uncertainty in estimating the worst possible case.
If it were necessary to depend wholly on the records of storms which have occurred in the United States, it might be thought possible for moderately great storms to occur over a period of a few hundred years, and then to find, as an exception, a storm three or four times as great. Theoretically that is very improbable, simply because water vapor in sufficient quantities cannot be transported from the ocean or gulf fast and long enough to cause such exceptional storms. As stated in chapter XI, however, records were collected of the stages of rivers in Europe for long periods of time, and these furnish fairly conclusive proof that such great exceptional storms actually do not occur. On the Danube at Vienna, for instance, we have records since about the year 1000 A.D.; fairly accurate records are available for stages of floods in the Tiber at Rome for more than 2,000 years; and records have been made of floods on the Seine at Paris for a long period of years.
Relation of Great Storms to Maximum Possible
After making the extensive investigation of storms in the eastern United States, it is believed that the March, 1913, flood is one of the great floods of centuries in the Miami Valley. In the course of three or four hundred years, however, a flood 15 or 20 per cent greater may occur. We do not believe a flood will ever occur which is more than 20 or 25 per cent in excess of that of March 1913. There is a factor of ignorance, however, against which we must provide, and the only way to do this is arbitrarily to increase the size of the maximum flood to be provided for. If longer records were available a closer estimate could be made, but in planning works on which the protection of the Miami Valley depends, it is necessary to go beyond human judgment. This has been done on all the other phases of the design, and we believe it would not be good engineering practice to stop at our judgment on this phase. We must be able to say that the engineering works are absolutely safe in every respect. For this reason provision is made for a flood nearly 40 per cent greater than that of March 1913. This is 15 or 20 per cent in excess of what is believed to be the greatest possible flood that will ever occur.Would modern tools cut the design margin due to reduced uncertainty or would they indicate that the project is now under-designed due to projected climate change? The latter seems unlikely considering that the magnitude of the purported effects has been repeatably shown to be smaller than we can reliably detect given the length of our data record. Would there be any practically significant changes to the decisions and designs? If your system already has sufficient margin for projected changes in weather-event magnitude do projected changes in frequency matter?
Reasons for Choosing as a Basis for Design a Flood 40% Greater than that of March 1913